4 sounds very young to me, but I'm not an orthodontist. Maybe there is a good reason?
Our orthodontist uses the general guideline that they are ready to start ortho when the front eight perm teeth are in (4 middle on top and 4 middle on bottom). My oldest started her palate expander around 7-1/2 yrs old. Our orthodontist tries to do most retainers and expanders at night, so hers was removable and she only wore it at night. We turned it once a week while it was out of her mouth and then she put it in. She had very little pain or discomfort and I think a large part of that is because she was so young. As kids get older, the palate turns from soft and pliable to hard bone. Once it is hardened, the palate expander is actually cracking the bone small amounts at a time and this is painful (I didn't get my palate expander until after I had gone through puberty and it was horrible). So, I definitely would advise looking into your options sooner, rather than later. However, unless something is drastically wrong, I would think that 4 is young.
Our orthodontist uses the general guideline that they are ready to start ortho when the front eight perm teeth are in (4 middle on top and 4 middle on bottom). My oldest started her palate expander around 7-1/2 yrs old. Our orthodontist tries to do most retainers and expanders at night, so hers was removable and she only wore it at night. We turned it once a week while it was out of her mouth and then she put it in. She had very little pain or discomfort and I think a large part of that is because she was so young. As kids get older, the palate turns from soft and pliable to hard bone. Once it is hardened, the palate expander is actually cracking the bone small amounts at a time and this is painful (I didn't get my palate expander until after I had gone through puberty and it was horrible). So, I definitely would advise looking into your options sooner, rather than later. However, unless something is drastically wrong, I would think that 4 is young.

Oh and eating certain foods was a problem. You can't get suction, food gets in between the roof of your mouth and the device. Was it bearable sure but it was no picnic. Good luck to the both of you.

I'm sure your dentist will refer you to an orthodontist who will decide the best time to start the process.
He referred ds 11 to an actual orthodontist as what my son needs it out of his realm of expertise.